Optically sensitive semiconductor devices have been known for many years and are the basis for a wide range of standard products in the semiconductor industry. Any optically sensitive semiconductor device relies on the fact that electromagnetic energy above a threshhold frequency is capable of creating electron-hole pairs in semiconductor materials. If these carrier pairs are generated near an unbiased or reverse biased PN junction the mobile carriers will migrate across the junction to produce a useful current and/or voltage.
The design of any PN junction structure intended to be used for photo detection must take into consideration several fundamental limitations. It is, of course, desirable to increase the surface area of the structure which is exposed to electromagnetic radiation. However, it is also necessary to minimize the distance which mobile carriers must travel before crossing the junction. Increasing this distance proportionately increases the likelihood that a carrier will be lost through recombination, thus decreasing the efficiency of the device. Finally, the depth of the PN junction structure should be optimized so that a large portion of the incident energy is absorbed in the active region of the structure.
These design considerations conjoin to make it particularly difficult to fabricate an efficient photo detector of the type commonly used as a part of an integrated circuit. Such devices are commonly fabricated in a tub of monocrystalline semiconductor material which is isolated from nearby devices by a surrounding layer of oxide. The fabrication of a relatively deep junction with a short average path length for carriers has been accomplished in the prior art by diffusing a dopant from the surface of the tub into its interior. The high temperature, long period diffusion step required to realize this structure is generally undesirable in terms of its effects on other devices on the die and for other reasons.